The critical dimension of semiconductor device is continuously decreased, such that the damage induced by any defect on a chip is continuously increased. Hence, high resolution examination (both review and inspection) is required to properly find and check the defect. For example, scanning electron microscope (SEM), as a more advanced examination system, is popularly used for examining chips. In general, a sample (wafer or photomask or semiconductor substrate) is located under a charged particle beam during the examination process. Clearly, whether the sample is correctly located is a key factor of the examining result. In general, one detector is located under the sample and on a base (such as the Z-stage of SEM). The detector will project or emit a light to the sample and analyze a reflected light from the sample. According to the analyzing result, the detector could acquire the geometric information (such as position, direction and angle) of the sample.
There are some drawbacks in the conventional design of the examination system. First, the detector is fixed on the base. Hence, once the base is improperly located (such as the top surface of the base is not parallel to the bottom surface of the sample located on a predetermined position), the detector usually can not properly detect the sample (such as the light is not properly projected from the detector onto the sample, such that the quality of the reflected light is degraded.) Second, only one detector is used to detect the geometric information of the sample. Hence, once the sample has a displacement around the light (or around an axis parallel to the light), the detector usually can not find the displacement.
Therefore, it is desired to develop some new designs of examination system to improve the above drawbacks.